1. Field of the Invention
This invention relates generally to integrated circuits, and more specifically to an integrated, on-chip resistor with a range of available, different resistance states. Also, this invention pertains to electronically programmable analog variable resistors and circuits using devices exhibiting electrically programmable, different resistances.
2. Related Art
In the current state of the art, there are no integrated circuit resistor elements that are programmable. In an integrated circuit, resistors are typically fabricated in silicon by doping the silicon to a level that fixes the material resistance. This resistance is unchangeable and irreversible in the final integrated circuit. Another way of achieving a resistance in an integrated circuit is by biasing a transistor, such as a MOSFET, at certain conditions. This technique requires constant power and is thus not a low-power, or no-power, passive resistance. With neither technique is it possible to create a range of available resistance states during integrated circuit operation, referred to as variable resistances, which retain their state in the absence of an applied electrical signal.
The lack of available passive variable resistor components prevents certain basic circuit functions that require analog or programmable resistors at the chip level. On-chip integrated, programmable analog resistors would enable, for example, field-programmable circuitry, reconfigurable electronics, reduction in size of the associated circuit electronics, and lower power operation.
Chalcogenide glasses containing an excess of metal ions, e.g. Ag ions, have been shown to exhibit negative differential resistance (NDR). In this context, NDR is the same as differential negative resistance (DNR)—referred to in the prior art. U.S. Pat. Nos. 7,329,558, 7,050,327, and 7,015,494 describe devices displaying NDR/DNR, using these devices in binary electronic memory, and as an analog memory via the current value read at the NDR peak. In the prior art, the NDR device is formed by addition of an excess of metal ions in a chalcogenide glass by either heating a chalcogenide material layered with a metal layer or by application of a fast electrical pulse with pulse width and amplitude specific to the chalcogenide material type. The peak current is programmed by application of a pulse of duration and amplitude that can cause the peak current to be either reduced or increased. The device current is read at or near the voltage corresponding to the peak current value.
NDR devices can be fabricated with standard complementary metal-oxide semiconductor (CMOS) processes at sizes consistent with the state of the art feature sizes, thus integrating well with existing and future integrated circuit technologies. Conventional chalcogenide devices may be comprised of GexSe1-x, wherein 0≦x≦0.9. Some chalcogenide devices contain copper and/or silver and/or mixtures thereof. For example, chalcogenide devices may comprise any combination of (GexSe1-x)yCu1-y and (GexSe1-x)yAg1-y, wherein 0≦x≦0.9 and wherein 0.1≦y≦0.9. Also, some chalcogenide devices are of a single layer of chalcogenide material containing an excess of metal which causes the NDR response. Chalcogenide devices exhibiting NDR can be used as continuously variable resistors by programming with material specific pulse conditions, and using the resistance of the device measured between 0V (zero volts) and the potential corresponding to the NDR peak current in an integrated circuit. The pulse conditions applied to the chalcogenide device determine the resistance value obtained. The typical range of resistance values is semi-continuous, and thus analog, within the MOhm range to the Ohm range.
U.S. Pat. No. 7,015,494 (Campbell '494) discloses an integrated device, for example, a tunnel diode, displaying differential negative resistance (DNR)—an operating region in which Ohm's classic relation between current and voltage (I=V/R) does not apply. The '494 device is a layered semiconductor construction on a substrate, the layers being:
1. Ge and one or more of S, Te and Se;
2. a transition metal and one of more of O, S, Te and Se; and,
3. Ge and one or more of S, Te and Se.
The disclosure of the '494 patent, invented by the subject inventor, is incorporated herein by reference.
U.S. Pat. No. 7,050,327 (Campbell '327) discloses a DNR memory device that can be programmed to store information as readable current amplitudes. The memory device is made of the material disclosed in the '494 patent, and the stored data is semi-volatile. The disclosure of the '327 patent, invented by the subject inventor, is also incorporated herein by reference.